1. Field of the Invention
The present invention relates to a method and apparatus for drying an applied film and, particularly, to a method for drying a long and wide applied film that is formed by applying an application liquid containing an organic solvent to a long support medium in the manufacture of an optical compensating sheet or the like, and a drying apparatus.
2. Description of the Related Art
In order to improve the viewing angle characteristics in a liquid crystal display, an optical compensating sheet is provided as a phase contrast sheet between a pair of polarizing sheets and a liquid crystal cell. A method of manufacturing long optical compensating sheets is disclosed in Japanese Patent Application Laid-Open No. 9-73081. Disclosed is a method that involves forming an oriented film by applying an application liquid containing an oriented film forming resin on a surface of a long transparent film and then performing rubbing treatment, applying an application liquid containing a liquid crystalline discotic compound to the oriented film, and drying the applied film.
In the method of drying an application liquid containing a liquid crystalline discotic compound disclosed in Japanese Patent Application Laid-Open No. 9-73081, initial drying is performed under indoor air-conditioned conditions for the duration from the application of an application liquid containing a liquid crystalline discotic compound to the oriented film to the drying by use of a regular drying apparatus, whereby mainly an organic compound contained in the application liquid is evaporated.
In an optical compensating sheet manufactured by this method, two types of surface irregularities A and B occur on an applied film surface 1 in the drying process as shown in FIG. 4, Type A being broad irregularities (indicated by thin lines) and Type B being sharp irregularities (indicated by thick lines), thereby posing the problem that this lowers the yield of products in some cases.
The two types of irregularities A and B were analyzed and as a result, it became apparent that in a broad irregularity A, as shown in FIG. 5, the thickness of the layer of an application liquid film 2 containing a liquid crystalline discotic compound has become small. In FIG. 5, reference numeral 3 designates a long support medium and reference numeral 4 designates an oriented film layer. On the other hand, it became apparent that, as shown in FIG. 6, the orientation direction 6 of an oriented portion 5 (a shaded portion) in which a sharp irregularity B occurs is displaced compared to other oriented portions 8 of a normal orientation direction 7.
As an effective measure against such irregularities A and B that occur in an initial drying, there is a method to prevent the occurrence of the irregularities that involves increasing the viscosity of an application liquid by raising the concentration of the application liquid or adding a thickener, thereby suppressing the flow of an applied film surface immediately after application by a drying wind. As another method, there is available a method that involves using a high boiling point solvent, whereby even in the case of the occurrence of the flow of an applied surface due to a drying wind immediately after application, the occurrence of irregularities is prevented due to the producing of the leveling effect.
However, the method that involves increasing the viscosity of an application liquid by raising the concentration of the application liquid or adding a thickener, has the disadvantage that it is impossible to perform ultrathin-layer precision application that involves forming an applied film of an ultrathin layer by high-speed application. Furthermore, this method has the disadvantage that because the higher the viscosity of an applied liquid, the lower the critical application speed (a limit to an application speed at which stable application is possible), the viscosity increases and high-speed application becomes impossible, thereby extremely deteriorating the production efficiency.
On the other hand, the method that involves using a high boiling point solvent brings about an increase in the drying time and an increase in the amount of a residual solvent remaining in an applied film and has the disadvantage that the production efficiency becomes worse because of a longer drying time.
As a method for solving these disadvantages, in Japanese Patent Application Laid-Open No. 2001-170547 are proposed a drying method and a drying apparatus which are such that a drying zone is provided immediately after application, an applied film surface of the traveling long support medium to be dried is enclosed, and a drying wind of a unidirectional flow that flows from the one-end side of the width direction of the long support medium to the other-end side thereof is generated in the drying zone, whereby an applied film can be uniformly dried without the need to change the physical properties, such as viscosity, of the application liquid and the kind of the solvent.